1. Field of the Invention
This invention relates to laser beam amplification and delivery apparatus and methods, and more particularly to the delivery of a high power near diffraction limited optical beam from a central station to one or more remote local stations.
2. Description of the Related Art
There are numerous potential applications for the transmission of high power laser signals from a central station to a number of separate local work stations. For example, a single laser unit could be used to support multiple factory work stations that perform welding and cutting operations. Such an arrangement would enhance overall productivity, since each work station is typically active only about 10%-20% of the time. Also, if multiple systems are multiplexed together, one laser unit can cover for a different laser unit that might be inoperative for maintenance. Another application could be the performance of laser surgery in different operating rooms of a hospital, supplied from a single laser source. One or more optical transmitting apertures integrated into the wing of an aircraft could also be supplied from a single central high power laser source.
To operate most successfully, the beam delivered to each local station should be near diffraction limited, i.e., with maximum collimation and a planar wavefront. Diffraction limited beams can be focused to a small spot or used over a greater working distance than poorer quality beams. For example, they make possible higher welding and cutting rates and a greater degree of application flexibility in the case of factory work stations.
To preserve a diffraction limited beam's quality, a laser beam would normally be transmitted over a single mode optical fiber. However, high power lasers generally require large, multi-mode fibers with core diameters typically on the order of 1 mm. Unfortunately, transmission through a multi-mode fiber progressively distorts the beam and degrades its diffraction limited quality. This has not been a problem with prior fiber optic beam delivery systems, since the high power lasers they employed produced beams that had very poor quality to begin with (typically 100-200 times diffraction limited). Accordingly, any additional deterioration in beam quality resulting from the use of multi-mode fibers was not significant. High power lasers capable of producing beams of much higher quality are presently being developed, however, and with these lasers the loss of beam quality resulting from transmission through multi-mode fibers would be a serious drawback.
The transmission of a high power laser beam can also be limited by attenuation from processes such as stimulated Brillouin scattering (SBS). The presence of SBS has generally not been a problem with previous fiber beam delivery systems, since the high power lasers employed typically operated with broad spectral widths and short coherence lengths (typically on the order of 1 cm). However, for beams produced by some advanced lasers that exhibit narrower bandwidths and longer coherence lengths, SBS can become a limiting factor in the system's power handling capacity. With remote work stations located at distances of 100 m or more from the central laser source, both degradation of beam quality and beam attenuation through SBS can be expected to be encountered.
Together with the supply of multiple local stations from a central laser source, the present invention is concerned with optical phase conjugation. The use of optical phase conjugation to compensate for distortions introduced while transmitting an image through a long multi-mode optical fiber has been known for some time. For example, see Yariv, "Three-Dimensional Pictorial Transmission in Optical Fibers", Applied Physics Letters, Vol. 28, No. 2, Jan. 15, 1976, pages 88-89, and Dunning, et al., "Demonstration of Image Transmission Through Fiber Optical Phase Conjugation", Optics Letters, Vol. 7, No. 11, November, 1982, pages 558-560. However, the systems described in these references employ phase conjugation apparatus that is remote from the laser source. This approach is incompatible with the desired grouping of the laser source and phase conjugation apparatus at a central station to avoid redundancies among multiple local stations. In Luther-Davies et al., "Single-Mode Resonator Incorporating an Internal Multimode Optical Fiber and a Phase-Conjugate Reflector", Journal of the Optical Society of America B, Vol. 7, No. 7, July 1990, pages 1216-1220, an optical fiber incorporated within a phase conjugate resonator is compensated to obtain a diffraction limited output from the fiber. However, this approach suffers from several disadvantages. First, the response time of the Luther-Davies et al. phase-conjugate mirror (PCM), which was based on the photorefractive effect, is relatively slow, and this limits the rate at which the fiber can be moved or flexed. For example, with the device described by Luther-Davies et al. rapid movements of the fiber cause the beam to be extinguished and to re-form after a few seconds. They report that when the fiber was flexed through 90 degrees, continuous operation of their device would only be achieved if the flexing occurred over approximately a five-second period. This represents a severe limitation in most anticipated applications such as robot welding arms, in which a time response on the order of milliseconds is required. Second, operation of their photorefractive PCM requires an auxiliary laser source in addition to the disclosed laser resonator. This requirement adds to the overall cost and complexity of the system. In principle the auxiliary laser could be eliminated by employing any of several self-pumped PCMs that have been reported. However, self-pumped PCMs are limited to reflectivities less than unity, and this low reflectivity can seriously reduce the efficiency of a resonator incorporating such a PCM. Third, for photorefractive PCMs to function it is essential that they absorb some fraction of the incident radiation to generate the charge carriers that lead to the desired photorefractive effect. This absorption is typically .about.0.1 percent; although this is a rather low value, it may lead to thermally induced performance degradations in high-power applications. Finally, photorefractive PCMs are not available at all wavelengths of technological interest such as the YAG laser wavelength of approximately one micron, which implies that the concept described by Luther-Davies et al. is only of limited applicability.
Phase conjugation has been used to obtain an amplified output beam from an amplification medium that is optically distorted. For example, in U.S. Pat. No. 4,757,268 to Abrams et al. and assigned to Hughes Aircraft Company, the assignee of the present invention, a low power source beam is transmitted through a plurality of laser gain elements, and then phase conjugated and transmitted back in the opposite direction through the same gain elements. Distortions imposed upon the beam during the initial pass through the gain elements are thus compensated during the reverse pass. While the system produces a high quality optical output, it does not deliver the output to a local station remote from the laser source, phase conjugation and amplification apparatus.
A self-aligning phase conjugate laser system is disclosed in U.S. Pat. No. 4,812,639 to Byren et al., also assigned to Hughes Aircraft Company, in which a laser oscillator is provided at a first location and in one embodiment communicates through an optical fiber with a laser amplifier and phase conjugate mirror at a separate location. For example, the laser oscillator may be incorporated into a surgical instrument that is hand held by a physician. While this system could be used to provide optical amplification and phase conjugate compensation from a central station to a number of remote surgical stations, a separate laser oscillator would be required at each local station. In addition to adding to the cost and complexity of the overall system, this would also increase the weight and bulk of the hand held surgical instrument.